The invention relates to electronic semiconductor devices, and, more particularly, to fabrication of silicon-based devices.
Silicon integrated circuits typically electrically isolate individual field effect transistors, bipolar transistors, and any substrate resistors and other elements with silicon dioxide ("oxide") regions at the surface of a silicon wafer. These oxide isolation regions can be directly formed by a thermal oxidation of a silicon wafer with an oxidation barrier such as silicon nitride ("nitride") masking off areas which will eventually contain transistors, substrate resistors, and other elements. This method of oxidation of selected regions of a silicon wafer has acquired the acronym LOCOS ("local oxidation of silicon"). See for example, Runyan and Bean, Semiconductor Integrated Circuit Processing Technology (Addison-Wesley 1990) pages 108-110.
Typical LOCOS includes using a thin oxide layer between the nitride mask and the silicon wafer; this oxide provides stress relief during the thermal oxidation. However, thermal oxidation of silicon proceeds essentially isotropically, and the oxidation encroaches under the nitride mask along the pad oxide to form an oxide wedge termed the "bird's beak". FIGS. 1-2 illustrate LOCOS with nitride mask 102 on pad oxide 104 which is on silicon wafer 106. FIG. 1 is prior to thermal oxidation and FIG. 2 is after thermal oxidation which forms isolation oxide 108. The bird's beak 110 growth warps nitride 102 and may also generate defects in the adjacent silicon wafer due to the stresses generated.
The bird's beak limits the scaling down and packing density of the devices in an integrated circuit. Thus attempts to reduce the extent of the bird's beak have been made and include making the pad oxide thinner and inserting a polycrystalline silicon ("polysilicon") layer between the nitride and the pad oxide ("poly buffered LOCOS" or "PBL"). FIGS. 3-4 illustrate a form of PBL with nitride mask 302 on polysilicon buffer 303 which is on pad oxide 304.
Removal of the nitride mask 102 after LOCOS thermal oxidation or nitride mask 302 after PBL thermal oxidation requires a nitride or a nitride plus polysilicon etch which will stop on the pad oxide and thereby avoid damaging the underlying device area silicon. The standard nitride etch uses a bath of hot phosphoric acid (H.sub.3 PO.sub.4) which is highly selective to oxide. However, wet etches introduce undesired contamination of a wafer for two reasons: liquids typically cannot be purified sufficiently and the wafer must be removed from the oxidation chamber for the wet nitride stripping (plus pad oxide removal and cleanup) and then reinserted into a processing chamber for subsequent steps, typically a thermal oxidation to form gate oxide. An all dry processing sequence for nitride stripping can avoid the wet etch and the removal/reinsertion contamination sources.
Nitride and polysilicon can also be used in other integrated circuit processing steps which require isotropic stripping. For example, vias in an oxide insulating layer can be filled by blanket polysilicon deposition followed by an etchback. Similarly, a wafer with a nitride backside seal and a frontside deposited protective oxide may require a selective nitride strip to avoid disturbing the frontside oxide.
Suto et al, Highly Selective Etching of Si.sub.3 N.sub.4 to SiO.sub.2 Employing Fluorine and Chlorine Atoms Generated by Microwave Discharge, 136 JECS 2132 (1989), report the selective etching of nitride with respect to oxide with the interhalogen compound ClF. Suto et al generated the ClF by Cl.sub.2 reacting with F atoms derived from an NF.sub.3 plasma.
Loewenstein et al, Chemical Etching of Thermally Oxidized Silicon Nitride: Comparison of Wet and Dry Etching Methods, 138 JECS 1389 (1991), compare methods of stripping the LOCOS nitride. Note that the nitride becomes oxidized at its surface to form a silicon oxynitride during the LOCOS thermal oxidation, and thus etch selectivity with respect to oxide may slow down the nitride etch but is necessary to stop on the underlying pad oxide. Thus there is a problem of efficiently stripping nitride selectively over oxide and polysilicon and of efficiently stripping polysilicon selectively over oxide and nitride.